Electron discharge device circuit



Aug.. 20, 1940. s. T. BREWER 2,212,337

I ELECTRON DISCHARGE DEVICE CIRCUIT I Filed Jan. 27, 1959 ATTOR/VPatented Aug. 20, 1940 PATENT OFFICE ELECTRON DISCHARGE DEVICE CIRCUIT ySherman T. Brewer, New York, N. Y., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication January 27, 1939, Serial No. 253,123

3 Claims.

This invention relates to electron discharge device circuits, and, moreparticularly, to circuits utilizing negative feedback.

It is known to use electron discharge devices orA tubes connected inbalanced or push-pull arrangement to-obtain a circuit from whose outputeven order harmonics are substantially eliminated. If it is desired tocontrol or vary the output of such an arrangement, it is necessary, ifthe even harmonic balance is to be maintained, that the gains of thetubes be equal to each other for all settings of the output control.` Iitwo ganged-potentiometers are used as an output cond trol, the necessityof maintaining a balance places a very severe tracking requirement onthe potentiometer. It would be preferable, therefore, for the outputcontrol to consist of a single variable impedance element which variesthe gains of the tubes simultaneously.

An object of the invention is to vary the output of a balanced orpush-pull circuit and yet maintain the harmonic balance of the output.

A feature of the invention comprises varying the output of a balanced orpush-pull circuit by varying the amount of local degenerative feedbackin said circuit.

Another feature of the invention comprises utilizing local degenerativefeedback for each tube of a balanced or push-pull circuit andsimultaneously varying the amount of degenerative feedback for each tubewithout affecting the balanced 'character'of the output of the circuit.

Afurther feature comprises regulating the gain of an ampliiiercomprising an odd number of electron discharge devices in tandem, inwhich' impedances are included in the common paths of lthecathode-control grid and cathode-anode circuits of the initial and theFinal devices, by varying an impedance connected between the cathodeends of the cathode impedances.

In one embodiment in accordance with the invention, a pair oi electrondischarge devices or tubes are connected in balanced or push-pullarrangement, the control grid-cathode and cathodeanode circuits or eachdevice having an impedance, preferably, a resistance, therein .providinga predetermined biasing potential for the control grid and a preassigneddegenerative feedback of signal. rThe output of the balanced cir cuit iscontrolled or regulated by simultaneously changing the gains of thetubes, the control being exercised through a variable impedance,preferably, a variable resistance, connected in shunt of theseries-connected cathode resistances and located'in the alternatingcurrent path only of the output circuit of the balanced circuit. Thevariable resistance may be of the type in which a slide or contact isadjusted in position to vary the amount of resistance effectivelyconnected or removed from the circuit with which it is associated; or,it may be of the type comprising a material having a non-linearcurrent-resistance characteristic, Whose resistance varies with theamount of current flowing therethrough, for ex'- ample, silver sulphide.

In another embodiment, in accordance with the invention, a plurality ofelectron discharge devices or tubes are connected in tandem, the initialand the iinal ldevice each having impedance means common to theircontrol grid-cathode and cathode-anode circuits, and having additionalimpedance means connected between the cathode tion which follows, takenin conjunction with l the appended drawing, wherein: Fig. 1 shows asingle-stage balanced or pushpull circuit embodying the invention;

Fig. 2 shows a circuit arrangement embodying theV invention forregulating the gain of an amplifer in a transmission line; and

Fig. 3v shows a multistage amplifier having an odd number of stagesembodying the gain control arrangement of the invention.

Fig. 1 shows a pair of electron discharge devices or tubes lil, I0connected in balanced or pushpull arrangement in a wave translatingcircuit that may comprise a single stage of an amplifier, an oscillatoror a detector. Each device comprises a cathode I2, I2', which may be ofthe indirectly heated type; an anode I3, I3; and an input control gridI4, I4. Each control grid is connected to a signal input terminal I5,and each anode is connected to an output terminal I6 and, through anoutput resistance Il, I'I, to the positive terminal of a suitable anodepower supply. A resistor i8, I8 is connected in the cathodecontrol gridand cathode-anode circuits of each tube. A variable resistor I9 isconnected in shunt of the resistors I8, I8 connected in series. Theresistor I9 may be of the type including a slide or a contact,adjustment of which varies the amount of resistance effectively in thecircuit, or it may be of the type in which the resistance varies withthe amount of current flowing therethrough, i. e.,

it may be one, or of a material, having a nonlinear current-resistancecharacteristic.

The resistors I8, I8 provide a predetermined biasing potential for thecontrol grids and, when signal is applied to the control grids, a localdegenerative feedback from the output to the input of the tubes. Theresistor I9 enables the gain of the push-pull circuit to be varied byvariation in the amount of degenerative feedback, Without affecting theeven harmonic balance obtainable with a push-pull arrangement of a pairof electron discharge devices.

With the arrangement described, if feedback in the tubes is set equal atone value of output b-y selecting appropriate values of resistance forthe resistors i8, I8', feedback in the tubes Will track automatically atall o-ther values as the resistance of the element I9 is varied. It willbe noted that there is no direct current potential across the resistorI9. This reduces contact noise inasmuch as with zero signal on thegrids, changes in the resistance of element I9 produce no output. Thedirect current operating potentials of the tubes are constant and areindependent of the setting of the feedback control. Although the lattermay be adjusted so that the feedback of the signal has been reduced tozero, there will be feedback to suppress any unbalance in even orderharmonics which might be generated.

Inasmuch as alternating current only ows through resistor I9, it ispossible to use a resistor of a material having a non-linearcurrentresistance characteristic that could not be used advantageouslyif direct current were to ow through it, for example, silver sulphide.With this material, the gain of the push-pull stage may be caused tovary automatically as. the load thereon varies from a preassigned normalvalue.

The gain control arrangement described with reference to Fig. l may beapplied to vary the gain in a negative feedback amplier containing anodd number of stages, for example, the threestage amplier shown inschematic in Fig. 3. The impedances Z4 and Z5 represent interstag'enetworks, and the impedances Z1, Z2 and Z3 comprise the beta circuit ofa negative feedback ampliier of the type described in H. S. Black Patent2,102.6'71, issued December 21, 1937. 'Ihe networks Z1 and Z2 aredesigned to provide the desired phase shift and gain around the feedbackloop, and Z3 is a variable lmpedance, preferably a variable resistance.for changing the gain of the amplifier. Although separate biasingresistors 20 and 29 are shown, the networks Z1l and Z2 could be designedto include the necessary resistance to provide the desired bias on thegrid of the respective stage. Assuming equal bias on thefirst and thefinal stages of the amplier, there will be no direct current potentialacross Z3. Variation in Z3 will Vary the amount of feedback around theamplier and vary its gain, without disturbing the direct currentoperating conditions.

Fig. 2 shows how the invention may be applied to a circuit arrangementfor regulating the gain of an amplifier in a transmission linetransmitting, for example, carrier frequency currents; The line 40 hastherein an amplifier 4|, shown schematically, Which may have threestages and be of the negative feedback type disclosed in H. S. BlackPatent 2,102,671, vissued. December 2.1, shuntv impedance pad 42 isshown connected in the beta circuit 43 of the amplifier 4l. One elementof the impedance pad comprises a resistance element 44 having a highnegative temperature coeiiicient of resistance..

The resistance element is controlled by a heater element 45. An increaseof current ow in the heater element decreases the resistance of element14 for decreasing the amount of feedback through the beta circuit 43 toincrease the gain of the ampliiier. A decrease in the current flowthrough the heater element produces an opposite action whereby the gainof the amplifier is decreased.

A regulator circuit 45 is provided for controlling the heater element 65to maintain the output from the amplifier substantially constant.

The transmission line transmits not only the carrier currents but also apilot current used for controlling the operation of the regulatorcircuit, the pilot current being affected in the same manner as thecarrier frequency currents by changes in the amplier output. -Theregulator circuit comprises a. line dl connected to the line lf3 on theoutput side of the amplifier ill; a lter d8 for passing the pilotfrequency only; a control circuit il@ embodying the gain controlarrangement of Fig. 1*; a detector circuit 58; an amplifier 5l; and aline 52 for connecting the output of the amplifier 5I to the heaterelement 45.

The control circuit 49 comprises an input transformer 53 the ends ofWhose secondary winding are connected through condensers to the controlgrids of the electron discharge devices or tubes 54, 54 connected inpush-pull arrangement. The anodes are connected to the ends of theprimary winding of an output transformer 55. The grid-cathode andcathode-anode circuits of each tube contain a biasing and degenerativefeedback resistor 56, 56', a variable resistance element 51 beingconnected in shunt of said cathode resistors connected in series. Theelement 5'! is of a material having a nonlinear current-resistancecharacteristic, and preferably of silver sulphide, a materialhaving ahigh negative temperature coeflicient of resistance.

For normal output of the line amplifier the control circuit is adjustedso that the current ow inthe heater element is of a value such that theamplifier has the desired normal gain. Should the gain of the lineamplier vary for any reason, the change in its output will be reflectedin the control circuit of the regulator through the Variation in thepilot frequency input thereto. If it is assumed that the output of theline amplifier decreases, the pilot frequency input to the controlcircuit decreases, the output of the control circuit is less, thecurrent flowing in the heater element is less, and the latter has adecreased eiTect on the resistance element in the pad in the betacircuit of the amplifier, causing the resistance element to increase inresistance and the amount of feedback to decrease whereby the gain ofthe amplier is increased.l

prising a control grid, a cathode and an anode, an anode circuit foreach device, means comprising an individual unbypassed impedance in eachanode circuit to control the potential difference between each grid andits associated cathode, and means comprising a variable impedanceconnected in shunt of said impedances, said shunt impedance being of amaterial having a non-linear current-resistance character- 10 istie.

2. The combination as claimed in thepreceddevice, and a variableresistor shunted across said resistors, said variable resistance beingof a material having a non-linear current-resistance characteristic.

' SHERMAN T. BREWER.

